I am not a very frequent flyer but during my recent trips I found following two tools really very useful:
TripIt is an unbelievably convenient way to manage your online itineraries. As Joel Spolsky discussed here, it’s really magical. Just forward your trip related emails to email@example.com and all done! It’ll automatically generate you a beautiful itinerary and send you back the URL of it. You don’t even need sign-up. Your email address will differentiate you from others.
I love this site because today I just had a very comfortable flight because of it. If you’re tall like me and have traveled in economy class, you can easily guess the pain I face in long flights :-)
SeatGuru can guide you what seats suit you best. For example, if you are a tall person, you definitely need some extra legroom and this site can tell you which seats have extra legroom in your aircraft. Not only this, it also lists out seats which you should beware of.